1. Field
The present invention relates to an electronic apparatus, a transmitter, an amplifier, and an amplifier control method.
2. Description of the Related Art
Envelope tracking (ET) is known as a technique that improves efficiency of power use in an amplifier included in a transmitting unit of an electronic apparatus, such as a mobile terminal or a radio relay apparatus. Examples of the envelope tracking include a technique that improves efficiency of power use in an amplifier by controlling a drain voltage in accordance with changes in the envelope of a transmission signal (drain voltage control (DVC)).
Specifically, drain voltage control is a technique that changes a drain voltage of an amplifier in accordance with the amplitude of an envelope of a transmission signal, and thereby allows the amplifier to operate consistently in a nearly saturated state and achieves highly efficient operation of the amplifier. Changing the drain voltage in accordance with the amplitude of the envelope involves use of an envelope tracking signal obtained by conversion of the envelope.
As a technique for controlling an amplifier, there has been proposed a technique that integrates an input signal in the previous cycle using an integrator, subtracts the resulting value from the current input signal, and controls an amplifier such that its output is proportional to a result of the subtraction. There has also been proposed a technique that determines a difference between an output of an amplifier and a predetermined reference value, and controls the amplifier using a value obtained by integrating the difference. Additionally, there has been proposed a technique that controls a direct-current voltage of an amplifier such that it is proportional to an envelope of a transmission signal.
For changing a drain voltage such that it is proportional to an envelope value of a transmission signal, an electronic apparatus needs to be equipped with a high-speed device that can respond to changes in envelope. Examples of such a device include a digital-to-analog converter (DAC) and an amplifier. Mounting such a high-speed device is costly and increases power consumption. Therefore, a device having a lower response speed will be used in practice.
When a drain voltage is made proportional to an envelope value, a peak value of an envelope tracking signal does not agree with that of the envelope. This results in an excess or insufficient supply of power at a peak time. To prevent an excess or insufficient supply of power at peak times, an amplifier may be controlled, using an offset, such that one peak value of an envelope agrees with that of an envelope tracking signal. However, even when the agreement of one peak value is achieved using an offset, the agreement of other peak values cannot be achieved and thus, an excess or insufficient supply of power occurs. Thus, an efficient supply of power cannot be achieved simply by making a drain voltage proportional to an envelope.
To control a drain voltage such that an efficient supply of power can be achieved, it is desirable that the following conditions be satisfied. A first condition is that a control signal is processed in accordance with a limit of the response speed of a device that performs drain voltage control. A second condition is that a control signal is processed in accordance with a nonlinear response characteristic of the device that performs drain voltage control. The nonlinear response characteristic is, for example, that there is a difference in the response speed of the device between rising and falling edges of power supply. A third condition is that to minimize unnecessary power output, a waveform of the output of an amplifier is kept as similar as possible to an envelope of an original transmission signal.
As a known drain voltage control technique for avoiding these constraints, there is an amplifier having a configuration illustrated in FIG. 8. FIG. 8 is a block diagram illustrating a known transmitter, which serves as an electronic apparatus, including an amplifier that performs drain voltage control.
In this known technique, an envelope of a transmission signal output from a transmission signal generator 901 is detected by an envelope detector 902. A nonlinear transducer 903 refers to a look-up table (LUT) 904 to convert the envelope into an envelope tracking signal. In the LUT 904, a response characteristic of a device that performs drain voltage control is recorded in advance. After the envelope tracking signal is band-limited by a low-pass filter (LPF) 905 and a change of the signal is slowed down, a DAC 906 converts the envelope tracking signal into an analog signal. In accordance with the resulting envelope tracking signal, a drain voltage controller 907 controls a drain voltage of an amplifying unit 910. A delaying unit 908 delays the transmission signal. A DAC 909 converts the transmission signal into an analog signal. The amplifying unit 910 amplifies the transmission signal using the drain voltage controlled by the drain voltage controller 907, and outputs the amplified transmission signal.
FIG. 9 illustrates an example of the LUT 904. The horizontal axis of the graph illustrated in FIG. 9 represents an envelope value, and the vertical axis of the graph represents an output value of the nonlinear transducer 903. A curve 911 represents the output value corresponding to the envelope value input in the LUT 904. A line 913 indicates a lower limit of the output value at which a source voltage can be controlled. A line 912 indicates a direct proportion between the envelope value and the output value. As indicated by the curve 911, the output value does not fall below the lower limit for the source voltage control even at the smallest envelope value. When the envelope value increases to reach a certain level, the envelope value and the output value agree with each other.
FIG. 10 illustrates an envelope conversion process using the LUT 904. The upper graph in FIG. 10 indicates a value of an input envelope 914. The lower graph in FIG. 10 indicates an envelope tracking signal 916 generated by using the LUT 904, and an envelope tracking signal 917 which allows a more efficient supply of power. A line 915 indicates a lower limit of an output value at which a source voltage can be controlled. The envelope tracking signal 917 is an envelope tracking signal that takes into account a limit of a response speed and a nonlinear response characteristic of a device and has peak values which agree with the corresponding peak values of the envelope 914.
Japanese Laid-open Patent Publication Nos. 2009-124476 and 8-102623, and Japanese National Publication of International Patent Application No. 2003-533116 are examples of related art.